if i understand you correctly that your existing install is of 1 * R500 which you want to extend by putting another AP 1*R510 and you would like to use unleashed code.
there is no major challenge and would do it this way...
1) login into R500, upgrade it unleashed code (found on support website). Configure IP, GW, SM the way you want. Once up, test it real to see if it working the way you want or not... if all good, go to the step 2.
2) Bring up R510, login into it. Make sure Unleashed software version you have is same as your R500.. if not upgrade/dowgrade appropriately and then proceed. Once it is on correct version as R500, simply configure the IP, SM, GW & DNS as it was done for R500. Save & submit. Connect to network. Within matter of minutes, if all good, R510 will join the R500 and form a unleashed WLAN network as per configuration in R500
Best of luck..